In order to reduce impure deposits of iron ore to commercially usable grades of iron, impure deposits of iron ore are generally concentrated and pelletized prior to reduction processing in blast furnaces. Pelletizing impure mineral deposits has grown into a very large industry since the end of World War II. Mineral ores of various kinds are pelletized for ore production but the process is most commonly with impure iron ores, such as taconite. Approximately 40 million tons of iron ore pellets are produced annually in the United States and another 30 million tons are produced in Canada. Other significant pellet production facilities exist in several other countries including Brazil, Australia, Turkey, India, Norway and Japan.
High grade iron ore deposits in the United States were severely depleted by the war effort during World War II. In order to continue to produce steel in blast furnace operations in the U.S., alternate sources of iron were needed to feed the blast furnaces. The University of Minnesota and a number of steel companies concentrated their efforts on developing technology to upgrade low grade magnetic ores, commonly called taconite, into an acceptable iron ore feed for these blast furnaces. Taconite, which is abundant in Minnesota's Iron Range, typically contains about 25% magnetic iron as compared to the roughly 50-70% iron content of some higher grade iron ores. In order to use taconite in place of the higher grade ores in commercial reduction processes, the iron content of the taconite needed to be concentrated.
The process for concentrating the iron in taconite evolved to include blasting the taconite and crushing it into particles small enough to liberate most of the grains of magnetite. The pulverized ore is then upgraded to an iron content in excess of preferably about 67% iron in a series of concentrating steps. The resulting mineral material is typically an aqueous slurry which is filtered or otherwise reduced to a moisture content of between about 9-10% by weight.
This material cannot be added directly to a blast furnace because the average particle size is so small, typically in a range of about 10-40 microns in diameter. Small particles such as these can plug a blast furnace. In addition, they are often lost as air entrained dust when fed directly into a blast furnace. It was believed, however, that this problem could be overcome by agglomerating the resulting mineral material. The need for some method of agglomerating this material subsequently led to the development of the iron ore pelletizing industry.
The commercial pelletizing or agglomeration process is generally a continuous process in which filtered mineral material is conveyed into balling drums or "disks" to form pellets. The rotating drum or disk causes the concentrated mineral material to roll into balls, typically called "green" or undried balls or pellets.
Green ball growth is somewhat similar to the growth of a snowball when it is rolled in wet snow. As the ball is rolled, successive layers are added as the ball grows to form a large ball. Seed pellets are initially formed from the mineral material by the rolling action of the drum. During commercial operation, pellets are typically screened at the drum discharge and the undersized pellets are recycled back into the drum as seed pellets until they have grown to form a ball having a diameter of about 1/2 inch (about 1.25 cm).
These green pellets are typically screened to remove pellet fines, dried at increasingly higher temperatures, and "fired" at a temperature of about 2400.degree. F. (1315.degree. C). When the pellets are fired, the iron grains grow together to form somewhat porous iron matrices which provide strength to enable the pellets to survive significant handling at shipping and receiving sites during transshipment.
Early in the development of the pelletizing industry, it was recognized that green pellets without "binding" agents were not suitable for subsequent processing steps. For example, the green pellets often broke during the balling process or during the initial stages of the drying process. Therefore, it became necessary to add a binding agent or "binder" to the moist mineral material fed into the balling drum. Many different additives were tested before it was determined that bentonite clay or "bentonite" would provide the binding strength required. Subsequently, bentonite became the standard balling additive or binder used in the pelletizing industry. Bentonite clay is typically added to the mineral material at rates of somewhere between about 10-25 pounds per long ton (2,240 pounds) of pellets.
Unfortunately, bentonite contains significant amounts of certain materials which shorten the useful life and lower the performance of blast furnaces. One of these materials is silica which is undesirable because excessive amounts of silica result in excessive amounts of slag which must be removed from blast furnaces during processing. The silica in bentonite also has the undesirable effect of melting and reforming into a glassy coating which can coat the surface of the iron particles within the pellet. This phenomenon adversely affects the ability of blast furnace reducing gasses to enter the pellets, thereby lowering blast furnace productivity. Bentonite is about 60% silica. Bentonite also contains other undesirable elements such as sodium and potassium. Sodium and potassium apparently react with the refractory linings of blast furnaces, thereby reducing the useful life of each furnace lining. In addition, these elements are believed to cause pellets to exhibit undesirable "swelling" when processed in blast furnaces.
Over the years, there has been intensive research to develop a binder that does not have these undesirable characteristics. Among the many inorganic and organic binders which have been tested are clays, paint rock, soda ash, limestone, lime, hydrated lime, iron sulfates, amines, amine carboxolates, animal proteins (e.g. dried blood), manures, cereal grains, flours, hulls, corn cobs, gelatins, glues, gums, humic acids, lignins, lignosulfonates, pulp, polyacroleins, polyacrylamides, polyamines, starch, sugar, surfactants, wood chips, wood flour, carboxymethylcellulose (CMC), molasses, corn syrup, graft copolymers of acrylic acid, possuolan, cement, tar, pitch, polyvinyl alcohols, dolomite, synthetic organic dispersants and high molecular weight substantially straight chain water-soluble polymers.
The complexity and difficulty of finding a practical and functional substitute for bentonite, however, has been demonstrated by the continued use of bentonite as a binder. Today, bentonite remains the principal commercial binding agent used in industry.
Progress has been made toward resolving the complex technical problems inhibiting the use of organic binders, however. See, for example, Haas, et al., "Effectiveness of Organic Binders for Iron Ore Pelletization" in Report of Investigations/1989, U.S. Dept. Interior, Bureau of Mines (1989). Sodium carboxymethylcellulose (CMC), used in conjunction with soda ash, or sodium carbonate (Na.sub.2 CO.sub.3), has proven to be an acceptable binder in some operations and continues to be used in several commercial operations today. Similarly, copolymers of sodium acrylate and acrylamide, used in conjunction with soda ash, also show promise as binding agents.
Efforts to use other binders, however, such as starch in particular, have not been favorably received. Modified native starch would appear to be an excellent candidate as a binding agent. Substantial supplies of native starch of a consistent quality are widely available at a relatively low cost, especially as compared to synthetically produced organic binders such as those mentioned hereinabove. Starches do not contain significant amounts of silica, sodium or potassium. In addition, starches are also believed to be relatively insensitive to variations in the "water chemistry" or ion concentration levels of the moisture contained in the concentrated mineral materials. Furthermore, modified native starches generally exhibit strong binding characteristics which are desirable in good binders.
Despite extensive testing of starch binders during the past thirty plus years, however, starch has yet to find commercial acceptability as a binder in the pelletizing industry. In spite of its broad availability, attractive cost, lack of undesirable constituents, general insensitivity to water chemistry, and strong binding characteristics, starch is generally believed to be unacceptable as a binder for pelletizing particulate mineral material. Some of the reasons why starch is believed to be an unacceptable binder, include the following negative characteristics of starch binders:
1. Starch binders generally result in excessive tackiness on the surface of "green " pellets. This allows excessive amounts of mineral concentrate fragments to collect on the surface of green balls when sufficient starch binders are added to maintain acceptable drop strength and dry compression strength at typical concentrate moisture levels. It is believed, but not relied upon, that starches do not readily retain water in the interior of the green balls. This is believed to result in unacceptably low green ball moisture content in the interior of the balls and unacceptably high moisture content on the surfaces which tend to be considered wet or tacky. PA0 2. Starches exhibit the unacceptable characteristic of encouraging rapid and uneven ball growth during balling operations. This is thought to be due to excessive tackiness on the surface of the balls which is characteristic of pellets made from mineral concentrates including starch base binders, and generally results in pellets which display poor strength characteristics. PA0 3. Pellets bound with starch generally have a rough surface exhibiting surface "cratering " and a surface characteristic commonly referred to as "orange peel". Such rough surface characteristics commonly result in unacceptable tonnage losses during transshipment due to abrasion between adjacent pellet surfaces.
Because of these and other problems associated with the use of starch binders to pelletize particulate mineral material, starch base binders are generally considered to be unacceptable in the art. A need has been demonstrated for an inexpensive organic binder for pelletizing mineral ores. Therefore, because of the attractive characteristics of native starch, discussed above, a need also exists for a starch base binder and a method of using native starch as a binder for particulate mineral material which will prove to be acceptable within the pelletizing industry. The present invention addresses these and other needs and problems associated with the formation and use of mineral ore pellets in the pelletizing industry. The present invention also offers other advantages over the prior art and solves other problems associated therewith.